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Keyboard Repair

The keyboard mechanism used in the Sol computer was manufactured by
Keytronic
corporation, which is still in business. The technology employed
by the keyboard is interesting. Unlike a lot of keyboards, which
used "ohmic" switches (basically, a piece of metal shorting together two
other pieces of metal), which can stop working due to oxidation or bits
of crud getting in the way of the electrons, the Sol keyboard used capacitive
sensing.

In capacitive sensing, the circuit measures how much capacitance there
is under each key plunger. The capacitor is formed by the copper
traces on the PC board. Normally, the plunger on a key is up high,
and there is a certain amount of capacitance formed by the gap between
the two copper areas directly beneath the key. When the key is depressed,
the plunger comes down. On the bottom of the plunger is a pieces
of foam with a clear pieces of plastic on the side near the traces, and
a piece of mylar on the side near the plunger. With this piece of
mylar pressed close to the two copper pads, the capacitance increases,
and the circuit can tell that the key is depressed.

You can read about detailed operation of the keyboard by studying
the schematic
for the keyboard and reading the theory of operation in
chapter 8 of the Sol Systems manual.

In all the pictures below, click on an image to get a close up view
of the action.

This page is mostly obsolete, as there appears to be no source of replacement
pads. Geoff Harrison has figured out a way to make his own, and has a nice
write-up on how he did it. Please see Geoff's web page on
how to repair a Sol-20 Keyboard.
This link
is another approach for the Apple Lisa, which also used a Keytronic keyboard.

What happens inevitably is that the foam under the keypad loses elasticity
(even if the keyboard is never used) and the foam rubber can even crumble.
The symptom is that some of the keys don't work when depressed, or work
only sporadically. What happens is that when the key is pressed,
the foam compresses and doesn't spring back.

You can actually disassemble the keyboard by unscrewing the couple dozen
little screws and remove the entire keyboard frame, leaving the bare PC
board exposed. After hooking everything up again, touch the metal
Sol case to discharge any static electricity you may be harboring.
Type on the PC board with your bare fingers and you can actually activate
the keyboard. Key bounce is a problem, though.

I've fixed a couple keyboards, and this way works for me. No doubt,
others have better techniques, so please share them with me.

The first step is to get replacement keypads. Don't scrimp and
just replace the pads of the keys that appear to be failing: replace all
of the keypads while you are at it. Until the middle of 2001, it
was possible to get replacement keypads directly from Keytronic for $0.10
per pad. Add in $7 for shipping, and it cost about $16 to replace
all the keypads. However, Keytronic doesn't sell them anymore.

There is another avenue for getting replacement pads. Someone tipped
me off that Sun type 5 keyboards used the same keypads that the 25 year old
Sol used. I bought two used Sun type 5 keyboards only to find out that
they don't. However, I have found that Sun type 4 keyboards do
have compatible pads. I bought one keyboard used for $5 at a local junk
shop, and I've since bought two others on EBay for $6 each, plus shipping.

In the Sun keyboards I've taken apart, there are two different color
pads: yellow, and gray. The gray ones appear to be a different density
of foam, and are used under the keys that get the most use: shift, space
bar, return, backspace, control, etc. I took pains to put those presumed
"heavy-duty" pads under similar keys on the Sol. At first I found
that those keys didn't function very well in the Sol keyboard, but after
pounding them heavily dozens of times, they seem to be working OK now.
Perhaps they would be best avoided. Here's a picture of the two types
of pads to be found in Sun type 4 keyboards:

Once you have gotten hold of some pads one way or another, it is time
to replace the ones in your Sol.

Disconnect the little ribbon cable from the motherboard that goes to
the keyboard, and unscrew the four screws holding the keyboard to the case.
Work someplace unlikely to have static -- like at a wood table on a wood
floor, not sitting on your couch.

Here is the keyboard after it is removed from the Sol, before disassembly:

Flip the keyboard over and unscrew the couple dozen tiny screws:

When you have removed them all, the entire keyboard frame should just
come off. I've heard others say that they pry off each keycap (watch
out for the leaping springs!) and doing things somehow from the other side,
but that seems like a lot of work (and you have to remember where each
keycap goes when you are done).

You should see a plastic disk under each key plunger. I depress
each key so that the plunger is more exposed.

Before depressing the plunger:

After depressing the plunger:

Each plastic disk is held in place in each of the four corners by a
slit in a corner post. I take a small flathead screwdriver and pry
underneath the buried plastic disk by one corner. The disk will flex
and pop out. Since these pads are trash, you don't have to be too
gentle about it. With practice it should only take about five seconds
per key.

Prying off a pad:

Plunger sans pad:

When you've cleared them all out, it is probably a good time to shake
out/vacuum/brush out any loose crumbly pad material that has just been
liberated. Then start snapping in the replacement pads. Make
sure the clear disc is near the plunger, and the mylar side (the silvered
one) is going to be in contact with the PC board. Snapping the replacement
pads back in is a little slower than taking them out, partly because you
must be more careful. Just flex each disk and make sure that all
four sides are being held; you should actually hear a click when it snaps
into place. Use a small screwdriver or tweezers to push each of the
sides and make sure they are held down by the "claws" of the plunger.
The whole thing takes about half an hour.

The rest should be obvious: put the keyboard frame back on the PC board,
screw in the couple dozen tiny screws, screw the keyboard to the case of
the Sol with the four screws, then finally connect the keyboard ribbon
cable back to the motherboard (J1 on the keyboard to J3 on the motherboard).